The present invention relates generally to semiconductor laser device, and in particular to a semiconductor laser device used as a pumping source for an optical amplifier.
With the proliferation of multimedia features on the Internet in the recent years, there has arisen a demand for larger data transmission capacity for optical communication systems. Conventional optical communication systems transmitted data on a single optical fiber at a single wavelength of 1310 nm or 1550 nm, which have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of such single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route, which resulted in an undesirable increase in costs.
In view of this, there has recently been developed wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system wherein a plurality of optical signals of different wavelengths can be transmitted simultaneously through a single optical fiber. These systems generally use an Erbium Doped Fiber Amplifier (EDFA) to amplify the data light signals as required for long transmission distances. WDM systems using EDFA initially operated in the 1550 nm band which is the operating band of the Erbium Doped Fiber Amplifier and the band at which gain flattening can be easily achieved. While use of WDM communication systems using the EDFA has recently expanded to the small gain coefficient band of 1580 nm, there has nevertheless been an increasing interest in an optical amplifier that operates outside the EDFA band because the low loss band of an optical fiber is wider than a band that can be amplified by the EDFA; a Raman amplifier is one such optical amplifier.
In a Raman amplifier system, a strong pumping light beam is pumped into an optical transmission line carrying an optical data signal. As is known to one of ordinary skill in the art, a Raman scattering effect causes a gain for optical signals having a frequency approximately 13 THz smaller than the frequency of the pumping beam. Where the data signal on the optical transmission line has this longer wavelength, the data signal is amplified. Thus, unlike an EDFA where a gain wavelength band is determined by the energy level of an Erbium ion, a Raman amplifier has a gain wavelength band that is determined by a wavelength of the pumping beam and, therefore, can amplify an arbitrary wavelength band by selecting a pumping light wavelength. Consequently, light signals within the entire low loss band of an optical fiber can be amplified with the WDM communication system using the Raman amplifier and the number of channels of signal light beams can be increased as compared with the communication system using the EDFA.
For the EDFA and Raman amplifiers, it is desirable to have a high output laser device as a pumping source. This is particularly important for the Raman amplifier, which amplifies signals over a wide wavelength band, but has relatively small gain. Such high output is generally provided by a pumping source having multiple longitudinal modes of operation. The Furukawa Electric Co., Ltd. has recently developed an integrated diffraction grating device that provides a multiple mode high output laser beam suitable for use as a pumping source in a Raman amplification system. An integrated diffraction grating device, as opposed to a fiber brag grating device, includes the diffraction grating formed within the semiconductor laser device itself. Examples of multiple mode integrated diffraction grating devices are disclosed in U.S. patent application Ser. Nos. 09/832,885 filed Apr. 12, 2001, 09/983,175 filed on Oct. 23, 2001, and 09/983,249 filed on Oct. 23, 2001, assigned to The Furukawa Electric Co., Ltd. and the entire contents of these applications are incorporated herein by reference. Where Raman amplification is used in a WDM communication system, the amplification gain characteristic of the Raman amplifier may be changed depending on the wavelength number of the input signal light. However, it is desirable for the Raman amplifier to have a relatively flat gain characteristic over a wide wavelength range including the entire wavelength range of the WDM signal. Such a flat gain characteristic is achieved by feed back control of the pumping laser output in the Raman amplification system. FIG. 19 is a block diagram illustrating a configuration of a Raman amplifier used in a WDM communication system and having a feedback control to provide a flat Raman amplification gain characteristic.
In FIG. 19, semiconductor laser modules 60a through 60d provide pump laser outputs. The laser modules 60a and 60b output laser beams having the same wavelength via polarization maintaining fiber 71 to polarization synthesizing coupler 61a. Similarly, laser beams outputted by each of the semiconductor laser modules 60c and 60d have the same wavelength, and they are polarization-multiplexed by the polarization-synthesizing coupler 61b. Each of the laser modules 60a through 60d includes an integrated semiconductor laser device such as those disclosed in U.S. patent application Ser. No. 09/832,885, Ser. No. 09/983,175, and Ser. No. 09/983, and, therefore, outputs a laser beam having a plurality of oscillation longitudinal modes to a respective polarization-multiplexing coupler 61a and 61b via a polarization maintaining fiber 71. In addition, the light output of each laser device is monitored by a photodiode (not shown) in order to allow control of the laser module outputs as will be further described below.
Polarization-multiplexing couplers 61a and 61b output polarization-multiplexed laser beams having different wavelengths to a WDM coupler 62. The WDM coupler 62 multiplexes the laser beams outputted from the polarization multiplexing couplers 61a and 61b, and outputs the multiplexed light beams as a pumping light beam to amplifying fiber 64 via WDM coupler 65. Signal light beams to be amplified are input to amplifying fiber 64 from signal light inputting fiber 69 via polarization-independent isolator 63. The amplified signal light beams are Raman-amplified by being multiplexed with the pumping light beams and input to a monitor light branching coupler 67 via the WDM coupler 65 and the polarization-independent isolator 66. The monitor light branching coupler 67 outputs a portion of the amplified signal light beams to a control circuit 68, and the remaining amplified signal light beams as an output laser beam to signal light outputting fiber 70.
The control circuit 68 controls the optical output of each of the semiconductor laser modules 60a-60d based on the monitored Raman amplifier output in order to maintain a flat gain characteristic of the Raman amplifier. More specifically, the control circuit 68 controls the driving current, and therefore the output, of each of the semiconductor laser devices in the laser modules 60a-60d, based on the Raman amplifier output. Thus, the control circuit 68 must also have information about the output level of each semiconductor laser device. As noted above, the optical output intensity of these semiconductor devices is monitored by a photodiode that produces a monitor current based on a portion of the light output detected from the back end (non emitting surface) of the laser device. However, the present inventors have discovered that for integrated diffraction grating laser devices, a stepwise or waving fluctuation in monitor current occurs with an increase in optical output of the laser device.
For example, FIG. 20A and FIG. 20B are diagrams showing the optical output (Lo) dependency of the monitor current (Im). With the optical output dependency of the monitor current shown in FIG. 20A, when the optical output exceeds a certain level, waving and fluctuation occur with an increase of the optical output. On the other hand, with the optical output dependency of the monitor current shown in FIG. 20B, when the optical output exceeds a certain level, the monitor current increases stepwise with an increase of the optical output. These fluctuations in the monitor current makes it difficult to know the instantaneous output intensity of the semiconductor laser devices thereby making it difficult to perform stable optical amplification control.
Accordingly, one object of the present invention is to provide a laser device and method for providing a light source suitable for use as a pumping light source in a Raman amplification system, but which overcomes the above described problems. Another object of the present invention is to provide a laser device that provides a smooth increase in monitor current for an increase in optical output thereby making the optical amplification control simple and easy.
According to a first aspect of the present invention, a semiconductor device and method for providing a light source suitable for use as a pumping light source in a Raman amplification system are provided. The device upon which the method is based includes an active layer configured to radiate light, a light reflecting facet positioned on a first side of the active layer, and a light emitting facet positioned on a second side of the active layer thereby forming a resonator between the light reflecting facet and the light emitting facet. A diffraction grating is positioned within the resonator along a portion of the length of the active layer, and a non-current injection area is formed along the diffraction grating so as to suppress injection current in the portion of the length of the active layer. The non-current injection area preferably has a length Li greater than a length Lg of the diffraction grating by an amount necessary to prevent scatter injection current from affecting the diffraction grating.
The non-current injection area may be an insulating film formed along the diffraction grating, an area along the diffraction grating where an electrode or contact layer is excluded, or a current blocking layer or impurity region forming a diode junction that blocks the injection current. An impurity region may also be used as a high resistance region for suppressing injection current. The non-current injection area may include an insulating film formed along the diffraction grating and having a slot portion that penetrates the contact layer to separate the contact layer into two parts. The laser device may further include a slot penetrating and electrode, a contact layer, and a cladding layer to bifurcate the electrode and contact layers thereby forming separate electrode structures. In this device the non-current injection area is one of the electrode structures to which no injection current is applied.
According to another aspect of the invention, a semiconductor laser module, an optical amplifier, a Raman amplifier, or a wavelength division multiplexing system may be provided with a semiconductor laser device having an active layer configured to radiate light, a light reflecting facet positioned on a first side of the active layer, and a light emitting facet positioned on a second side of the active layer thereby forming a resonator between the light reflecting facet and the light emitting facet. A diffraction grating is positioned within the resonator along a portion of the length of the active layer, and a non-current injection area is formed along the diffraction grating so as to suppress injection current in the portion of the length of the active layer.